VIII.] CHEMISTKY OF DOLOMITES AND GYPSUMS. 85 



markable for the predominance of alkaline salts, which some- 

 times amount to one thousandth, or to more than one half the 

 solid matters present. These waters are distinguished from 

 the river- waters just mentioned by their comparatively small 

 amounts of silica and earthy carbonates, and by the presence 

 of a notable proportion of berates.* , 



"We may here refer to the strongly alkaline waters furnished 

 by the artesian wells of Paris and London as evidences of the 

 abundance of alkaline carbonates in natural waters, and to the 

 springs of Yichy and Carlsbad, the latter of which, according 

 to the calculations of Gilbert, furnish annually more than thir- 

 teen millions of pounds of carbonate of soda. The evaporation 

 of these alkaline waters, whether of rivers or of springs, must 

 give rise to natron-lakes like Lake Van and those of the plains 

 of Araxes, Lower Egypt, and Hungary. (Bischof, Lehrbuch, 

 II. 1143.) 



The carbonate of soda contained in these waters has its 

 source in the decomposition of feldspathic minerals, and shows 

 the continuance in our time of a process whose great activity 

 in former geologic ages is attested, as I have elsewhere main- 

 tained, by vast accumulations of argillaceous sediments de- 

 prived of a large portion of their alkali, and also by the car- 

 bonate of lime which, by the intervention of carbonate of soda, 

 has been formed from the chloride of calcium of the primeval 

 ocean and deposited as limestone (ante, pages 2 and 10.) 



An indispensable condition for the precipitation of carbon- 

 ate of magnesia is the absence of chloride of calcium from the 

 solutions, and this, in the presence of an excess of sulphates, 

 is attained simply by evaporating to the point where gypsum 

 becomes insoluble. In nearly all river and spring waters bicar- 

 bonate of lime is present in a large proportion, and is often the 

 most abundant salt. We have shown that, when mingled with 

 a solution containing sulphate of magnesia, it gives rise, by 

 double decomposition, to bicarbonate of magnesia and sulphate 

 of lime. By the evaporation of such a solution, the latter salt, 



* For descriptions of these various waters of Canada, see the following 

 essay in this volume. 



